Downloads folder . . . I mean stack
Apple calls them "stacks," but I don't know if I'll ever call them anything but folders. These are the new icons in the Dock, to the right of the dividing line. Apple has put the Documents and Downloads folders into the Dock; files you download from Safari or Mail automatically go into the Downloads folder/stack. The icon changes to display the last thing you put in it. The stack expands upward in a sexy way, displaying all the files enclosed. The same folders are in your Home folder.
You can, of course, remove these folders/stacks from the Dock with no harm done, since the real folders are in your Home window. I've got 5.2 gigabytes of files in my Documents folder on my Tiger machine[md}I hate to see what that will look like in a stack!